This invention relates to a method of making a retroreflective article having controlled divergence, and articles made by the method.
It is well known that retroreflective articles can be made from an array of microcube corner elements. Such an array of microcube corner elements can be made by ruling a master of “male” cube corners into a planar surface of a plate. This is taught generally by Stamm U.S. Pat. No. 3,712,706; and also is taught in detail in Pricone U.S. Pat. No. 4,478,769. Each of these patents is incorporated herein by reference in its entirety.
U.S. Pat. No. 4,478,769 describes a well-known method of making triangular cube corner elements, in which the planar surface of a master plate is ruled with a diamond cutting tool that cuts a series of precise parallel vee-grooves. To rule equilateral triangular cube corners, three sets of parallel grooves intersecting one another at angles of 60° are made; each groove also will have an included angle of substantially 70.53° disposed symmetrically, and will be ruled to a groove depth determined by the height of the cube corners desired. This method automatically results in an array of pairs of oppositely oriented equilateral triangular microcubes on the face of the master. To rule non-equilateral triangle cube corners the grooves within the parallel sets will contain angles other than 70.53°, and intersect at angles other than 60°, as disclosed, for example in Rowland U.S. Pat. No. 3,384,348. Methods for ruling non-triangle cube corners generally do not use three sets of parallel symmetrically disposed vee-grooves, but the faces of the cube corners are nevertheless formed from the walls of grooves, as disclosed, for example in Nelson U.S. Pat. No. 4,938,563.
The ruled master may then be used to make a series of duplicates, such as by electroforming, and the duplicates are assembled together to form a single “mother” tool. The assembled “mother” tool is used to electroform molds, which then can be assembled into a tool capable of providing microcube retroreflective elements on a web of plastic sheeting material such as by embossing, casting, or other methods known in the art.
Microcube corner retroreflective sheeting such as made by the method described above is used in highway safety applications such as highway signs and pavement markers. In such applications, the microcube corner elements retroreflect light from a vehicle's headlights back to the driver of the vehicle. This is an inexact retroreflection in which the divergence angle, α, ranges between approximately 0° and more than 3°. The value of α operative in any given situation depends on the geometry of the vehicle and the distance from the vehicle to the retroreflective material. For example, the divergence angle α for a large truck's right headlight and its driver at a distance of about 40 meters from a road sign will be approximately 3°, while the divergence angle α for an automobile's left headlight and its driver at a distance of about 600 meters from a road sign will be approximately 0.05°.
Also associated with the divergence angle, α, is a rotation angle, ε, which is a measure of the direction of the divergence. The value of ε will be different for left and right headlights of a vehicle, and will also depend on the vehicle geometry and the position of the road sign.
Ideally, microcube corner retroreflective sheeting used in road signs will produce a pattern of retroreflected light having sufficient intensity over a range of divergence angle values and rotation angle values. For example, even a non-urban retroreflective highway sign should retroreflect light through a divergence angle α of about 1°, which corresponds to the value of α from a large truck's right headlight back to its driver at a distance of about 120 meters from the road sign.
Improvements in the precision with which microcube corner elements can be ruled in a master plate and duplicated by embossing have led to concerns that such microcube corner retroreflective sheeting may be retroreflective over only a very narrow range of divergence angle, such as about 0.0–0.5 degrees, as well as narrow ranges of rotation angle. It would be preferred to provide a ruled array with cube corners producing the entire desired range of divergence and within very short distances on the ruled array.
Light that is retroreflected by micro-sized cube corner elements will experience a certain amount of diffraction because of the very small size of the microcubes. Such diffraction will result in retroreflection over broader ranges of both divergence angle and rotation angle. The particular ranges of α and ε will depend on the particular diffraction pattern of a given microcube, which will depend in turn upon the cube size, cube shape, the index of refraction of the cube material, and upon whether or not the cube faces have been metallized. Diffraction, however, is not a desirable method to enhance retroreflection through broader divergence and rotational angle, because the very small microcubes that achieve greater diffraction also cause a substantial quantity of light to be retroreflected with a divergence angle α of greater than about 3°, where the light is not useful to the vehicle driver. This is summarized in Table 1.
Table 1 indicates the spreading of retroreflection due to diffraction. Acrylic equilateral triangle cube corners are used in each case. The millimeter dimension measures the edge length of the triangle (identically 2.449×the cube depth, or 1.155×the ruling spacing). The percentages indicate how much of the total retroreflected flux is within a 1°, 2°, or 3° maximum observation angle. For example, for the triangle cube corner with side 0.05 mm only 27.9% of the total retroreflected light arrives between 0° and 1° observation angles.
TABLE 1Diffraction spreading of retroreflection from differentsize triangle cube corners0.4 mm0.2 mm0.1 mm0.05 mm0° to 1°91.6%82.5%66.7%27.9%0° to 2°95.7%91.6%82.4%66.6%0° to 3°97.1%94.4%88.9%79.1%
Diffraction results in idiosyncratic patterns which are unlikely to distribute the retroreflected light in a manner that will be most useful to a vehicle's driver. This is shown in FIGS. 4A–D.
It is known in the art to create intentional aberrations in cube corner elements by causing the dihedral angles of the cube corner elements to deviate slightly from 90°. The classic paper “Study of Light Deviation Errors in Triple Mirrors and Tetrahedral Prisms,” J. Optical Soc. Amer., vol. 48, no. 7, pp. 496–499, July, 1958 by P. R. Yoder, Jr., describes the well-known spot patterns resulting from such aberrations.
U.S. Pat. No. 3,833,285 to Heenan, assigned to the common assignee and incorporated herein by reference in its entirety, teaches that having one dihedral angle of a macro-sized cube corner element greater than the other two results in extended observation angularity in macrocubes, and specifically that the retroreflected light diverges in an elongated pattern.
U.S. Pat. No. 4,775,219 to Appledorn discloses retroreflective articles having tailored divergence profiles, wherein the cube corner elements are formed by three intersecting sets of parallel vee-grooves, and wherein at least one of the sets includes, in a repeating pattern, at least two groove side angles that differ from one another.
U.S. Pat. No. 6,015,214 to Heenan et al., assigned to the common assignee, teaches methods of forming microcubes by ruling vee-grooves into the edges of a plurality of flat plates, and discloses that the tilt angle of a cutting tool with respect to the surface of the edges being ruled can be adjusted continuously as each groove is cut as a function of the distance traveled by the cutting tool along the ruled surface.
It is thus one object of the invention to provide an article comprising an array of retroreflective microcube corner elements having controlled broader divergence.
It is another object of the invention to provide methods for making such an article.